Re: [PATCH] btrfs: fix uninitialized warning in btrfs_log_inode

[Qu Wenruo <quwenruo.btrfs@gmx.com>]

On 2023/5/17 17:13, Anand Jain wrote:
> On 17/5/23 15:46, Qu Wenruo wrote:
>>
>>
>> On 2023/5/16 09:34, zhangshida wrote:
>>> From: Shida Zhang <zhangshida@kylinos.cn>
>>>
>>> From: Shida Zhang <zhangshida@kylinos.cn>
>>>
>>> This fixes the following warning reported by gcc 10 under x86_64:
>>
>> Full gcc version please.
>> Especially you need to check if your gcc10 is the latest release.
>>
>> If newer gcc (12.2.1) tested without such error, it may very possible to
>> be a false alert.
>>
>> And in fact it is.
>>
>
> I also noticed that last_range_start is not actually uninitialized.
> However, it is acceptable to initialize a variable to silence the
> compiler if necessary, right? We have done something similar in the
> past.

I tend not to. Uninitialized variable warning itself is a good indicator
to let compiler help us to expose branches we didn't consider.

Without a no-brainer "int whatever = 0;" we may even hit bugs that some
corner cases may even use that initialized zero, but we didn't even
expect to go.

Thanks,
Qu
>
> Thanks, Anand
>
>> @first_dir_index would only be assigned to @last_range_start if
>> last_range_end != 0.
>>
>> Thus the loop must have to be executed once, and @last_range_start won't
>> be zero.
>>
>> Please do check your environment (especially your gcc version and
>> backports), before sending such trivial patches.
>> Under most cases, it helps nobody.
>>
>> Thanks,
>> Qu
>>
>>>
>>> ../fs/btrfs/tree-log.c: In function ‘btrfs_log_inode’:
>>> ../fs/btrfs/tree-log.c:6211:9: error: ‘last_range_start’ may be used
>>> uninitialized in this function [-Werror=maybe-uninitialized]
>>>   6211 |   ret = insert_dir_log_key(trans, log, path, key.objectid,
>>>        |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>>   6212 |       first_dir_index, last_dir_index);
>>>        |       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>> ../fs/btrfs/tree-log.c:6161:6: note: ‘last_range_start’ was declared
>>> here
>>>   6161 |  u64 last_range_start;
>>>        |      ^~~~~~~~~~~~~~~~
>>>
>>> Reported-by: k2ci <kernel-bot@kylinos.cn>
>>> Signed-off-by: Shida Zhang <zhangshida@kylinos.cn>
>>> ---
>>>   fs/btrfs/tree-log.c | 2 +-
>>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/fs/btrfs/tree-log.c b/fs/btrfs/tree-log.c
>>> index 9b212e8c70cc..d2755d5e338b 100644
>>> --- a/fs/btrfs/tree-log.c
>>> +++ b/fs/btrfs/tree-log.c
>>> @@ -6158,7 +6158,7 @@ static int
>>> log_delayed_deletions_incremental(struct btrfs_trans_handle *trans,
>>>   {
>>>       struct btrfs_root *log = inode->root->log_root;
>>>       const struct btrfs_delayed_item *curr;
>>> -    u64 last_range_start;
>>> +    u64 last_range_start = 0;
>>>       u64 last_range_end = 0;
>>>       struct btrfs_key key;
>>>
>